This invention relates generally to non-destructive testing of component materials subject to degradation and, more particularly, to a non-destructive monitoring of aircraft engine components for possible degradation.
In the upkeep and maintenance of most mechanical and/or electrical apparatus, repair and/or replacement of parts does not occur until a failure causes the apparatus to be inoperable. At that time, inspection is made to determine the particular failure that has occurred, and a replacement part is installed to bring the apparatus to an operable condition.
In contrast, there are certain types of apparatus which, because of safety concerns, are preferably not permitted to have their components reach the level of failure. An aircraft gas turbine engine is such an apparatus. Here, it has become common practice to predict, on the basis of component life histories the operating life of a component and, to periodically repair or replace such a component prior to the time in which it is predicted to fail. In this manner, a useful life of the component is approximated while minimizing the risk of failure.
In a turbine engine, component cracking (e.g. creep, low cycle fatigue (LCF), high cycle fatigue (HCF), stress corrosion cracking is usually associated with high stress risers (i.e. radius, bolt holes, flanges, etc) high temperatures, processing defects or combinations thereof. These stress locations can be identified by analysis or by experience from field failures. To mitigate risk from cracking, service life limits are determined for many components such as disks, blades, shafts, air seals, and tubing, and are removed from service before long cracks have a chance to evolve.
In the field of fracture mechanics, electrical potential difference is an established laboratory technique for determining crack growth rate characteristics in electrically conducting materials. The electrical field in these specimens is disturbed by the initiation of a crack and varies predictably with increasing crack size. In a case where constant current is imposed through the specimen, the potential drop across the crack plane will increase with increasing crack size. This predictable response to the electrical field is used to relate a change in voltage to crack size and is used as an automating means of continuously monitoring crack size.